In an incandescent lamp equipped with a filament, such as tungsten filament, the resistance of a non-lighted filament is extremely lower, generally, about one-tenth, than that in an incandescent state. For example, the resistance of a 100 watt incandescent lamp is about 100 ohms when lighted, whereas its resistance is less than 10 ohms when non-lighted. Since the peak magnitude of ac 100 volt lamp wire goes up to 141 volts, the incandescent lamp inevitably receives a 14 amperes of inrush-current when it is coupled with the lamp wire at the peak magnitude. Accordingly, such inrush-current would be a major factor of causing filament snapping.